This invention relates to a solid imaging device with a charge-coupled device (CCD) used for a charge transfer region transferring signal charges generated by a photoelectric conversion, and more particularly to a method for fabricating a solid imaging device having a structure for improving a smear and a breakdown voltage characteristics.
In a solid imaging device, an incident light energy is converted into an electric signal to generate a signal charge to be transferred to an output region through the charge transfer region for subsequent conversion into a voltage signal to be output of this device. The solid imaging device has a problem in phenomenon so called smear. The phenomenon due to the smear is like that the incident light energy which entered in and around the charge transfer region is converted into the electric signal, by which a generated charge mixes with the signal charge to obtain an alias.
A conventional method for fabricating the solid imaging device is described with referenced to the accompanying drawings.
Referring to FIG. 1A, an n-type silicon substrate 1 is prepared to form the solid imaging device. A boron (B) is implanted into the n-type silicon substrate 1 by use of an ion implantation method to form a first p-type well layer 2 by a heat treatment at high temperature.
Referring to FIG. 1B, a photoresist is applied on the first p-type well layer 2 to implant ions of a p-type impurity and an n-type impurity into the first p-type well layer 2, thereby forming a second p-type well layer 3, an n-type semiconductor layer 4 to be used as the charge transfer region and an n-type semiconductor layer 5 to be used as the photoelectric conversion region, respectively. The p-type well layer 3 is provided for prevention of a punch-through between the n-type semiconductor layer 4 and n-type semiconductor layer 5 and for control of a channel potential in the n-type semiconductor layer 4.
Referring to FIG. 1C, a silicon oxide film 6 is formed by use of a thermal oxidation method to be used as a gate silicon oxide film. After that, a polycrystalline silicon film is deposited on the silicon oxide film 6 to be subjected to a patterning to define a first charge transfer electrode 9. Subsequently, the photoresist and the first charge transfer electrode 9 are used as a mask, thereby implanting boron (B) into the n-type semiconductor layer 5 to form a shallow p-type semiconductor layer 10 for separating the n-type semiconductor layer 5 from the silicon oxide film 6 to reduce a dark current in the surface of the n-type semiconductor layer 5. After that, the exposed silicon oxide film 6 is etched by use of the first charge transfer electrode 9 as a mask.
Referring to FIG. 1D, a gate silicon oxide film is formed on a surface of the device by use of the thermal oxidation method, after which a second charge transfer electrode (not shown) is formed by deposition of a surface of the polycrystalline silicon film and carrying out a patterning. Subsequently, a interlayer insulator 12 is formed on a surface of the charge transfer electrode 9 by use of the thermal oxidation method.
As the result of the above step that forms the gate oxide film and the interlayer insulator film 12 by use of the thermal oxidation method, the end of the first charge transfer electrode 9 is bent upward due to the formation of the oxide film under the first charge transfer electrode 9. If the state of the above first charge transfer electrode 9 is maintained, a coverage is changed to the worse in forming a shield film 16. Thus, there is a possibility to have the rest of etching occur when the shield layer is subjected to a patterning, and also a breakdown voltage between the charge transfer electrode and the shield film is decreased.
For solving the above issues, a second layer insulation film 15 is formed for filling up a difference in level of the first charge transfer electrode 9 to the silicon substrate as shown in FIG. 1E. Subsequently, the shield film 16 for preventing an incident light to enter into all surface of the device except for the photoelectric conversion region is formed as shown in FIG. 1F.
The conventional method for fabricating the solid imaging device is engaged with disadvantage as mentioned below. In the conventional method, for instance, the interlayer insulator 12 having a thickness of 300 to 400 nanometers is formed on the charge transfer electrode for maintenance of a high breakdown voltage and filling up the difference in level of the charge transfer electrode. Further, in the conventional method, the interlayer insulator 15 having the same thickness as that of the interlayer insulator 12 is also formed on the interlayer insulator 12. Accordingly, a total thicknesses of the interlayer insulator 15 and the gate silicon oxide film formed prior to forming of the second charge transfer electrode provides a long distance between the silicon substrate and the shield film 16. The long distance between the silicon substrate and the shield film 16 results in the increase of the incident light to enter into all surface of the device except for the photoelectric conversion region. Therefore, the smear level is also increased and an output of the solid imaging device includes errors.
Further, the conventional method for fabricating the solid imaging device is also engaged with disadvantage as mentioned below. A leakage current is increased in the silicon oxide film formed by thermal oxidation of the surface of the polycrystalline silicon film (the first charge transfer electrode 9) because of including large crystal grains of a silicon in the silicon oxide film as compared with a silicon oxide film formed by thermal oxidation of a normal silicon. Such silicon oxide film has a poor breakdown voltage. The polycrystalline silicon film having a thickness of approximately 400 nanometers may permit the silicon oxide film to have the required breakdown voltage. Although the thick polycrystalline silicon film may have the silicon oxide film free from the issue of the breakdown voltage, the smear level is increased as explained above.